With advances in design, device components are being placed closer together in smaller-sized packages. As a result, traditional techniques for testing devices can be difficult to implement.
The Joint Test Action Group (JTAG) was formed by manufacturers to address testing problems associated with smaller devices. The findings and recommendations of JTAG were used as the basis for the Institute of Electrical and Electronic Engineers (IEEE) Standard 1149.1, “Standard Test Access Port and Boundary-Scan Architecture.” This standard is commonly referred to as JTAG. Boundary scan testing as defined by JTAG provides the ability to set and read values on device pins without direct physical access. According to JTAG, there are a number of registers associated with boundary scan testing. One of these registers is the Bypass register, which is a single-bit register that passes information from the “test data in” (TDI) to the “test data out” (TDO) pin. Devices under test can be can be coupled in series in what is referred to as a “scan chain.” The Bypass register allows the testing of other devices in a scan chain without unnecessary overhead.
However, testing of devices coupled in a scan chain can be problematic when one of the devices is powered off or is inoperable for some reason. When this occurs, there is a break in the scan chain and test signals cannot be passed down the chain. Therefore, with conventional scan chain testing, the chain needs to be intact, with all devices in the chain powered on and operable. Conventionally, the testing is halted until the scan chain is again intact; that is, testing is not performed until the device that is powered off or inoperable is either powered on, returned to an operable state, or physically removed from the chain. As a consequence, testing is made more difficult, and the time needed for testing is increased. Accordingly, a system and/or method that can allow scan chain testing when one or more of the devices in the scan chain are powered off or otherwise inoperable would be valuable.